Flat panel display devices, e.g., liquid crystal display devices, are widely used as those for personal computers, word processors, television receivers, etc. and projection type display devices as well because the display devices have the advantage of thin thickness, light weight or low power consumption.
Since active matrix type liquid crystal display devices, in particular, provided with switching elements connected to pixel electrodes have an excellent display without any substantial cross-talk between adjacent pixels, they have been vigorously researched and developed.
In manufacturing the flat panel display device, the snapping or short circuits of wirings or electrically conductive patterns and display defects, such as uneven brightness or chromaticity caused by dusts and/or partial defects, are inspected at the completion of a display panel unit or display module of the flat panel display device. In general, test signals are applied from signal line and scanning line pads to observe image display conditions of pixels for the detection of such defects as set forth above. Since these inspections are carried out during the time when a light source is turned on, these are called “turned-on tests”.
There have been various disclosures as to an automatic image processing system provided with a CCD camera for such turned-on tests (see, e.g., Japanese Laid-Open Patent Application Tokkaihei 8-145848 and Japanese Laid-Open Patent Application Tokkaihei 11-101712). It is easy to detect and judge the snapping or short circuits of wirings and electrically conductive patterns or black spots (disabled pixels) but relatively difficult to judge if uneven brightness (unevenness in light and shade) in spot regions or the like is within the tolerance. Thus, the automatic image processing system at present is second only to visual tests by skilled inspectors in the efficiency and accuracy of detection and/or judgment for subtly uneven display.
The assessment of uneven brightness such as its degree (density) and number appearing on the display panel is of critical importance as to the determination of whether the flat panel display device is good and in shippable quality or not. Inspectors are required to be highly skilled in such assessment while less skilled inspectors are often commit assessment errors.
The visual tests for the determination of uneven brightness is carried out in one of the following ways:
(1) One display panel with certain uneven density slightly exceeding the tolerance (a boundary sample for a bad quality panel) or another with unevenness barely within the tolerance (a boundary sample for a good quality panel) are used for comparison standards of reference The determination as to whether a display panel under inspection is good or bad is made in comparison of such a display panel with those samples.
(2) A neutral density (ND) filter with specific transmittance is placed on uneven display portions of a display panel under inspection to check whether any uneven display can be seen or not. If no substantial unevenness is seen, the display panel is judged to be good. Otherwise, it is judged to be bad.
(3) In order to display specific density unevenness an inspection signal is supplied to a display panel under inspection from an inspection device for the “turned-on tests”. In other words, the display panel is driven to display image patterns similar to unevenness by using software programs. The inspector can determine if the uneven display by the display panel is within the limit of tolerance for density.
Patent publication reference No. 1: Japanese Laid-Open Patent Application Tokkaihei 8-145648.
Patent publication reference No. 2: Japanese Laid-Open Patent Application Tokkaihei 11-101712.
The inspection methods of unevenness set forth above have the following problems:
(1) The inspection method by means of boundary samples
First, appropriate samples with specific uneven densities are rarely found.
Secondly, two independent sets of flat panel display devices are necessary to operate a display panel under inspection and a boundary sample at the same time for the benefit of efficient inspection. Where, however, only one set of the flat panel display device is available, the boundary sample and the display panel under inspection are driven separately or alternatively to compare the former with the latter while relying on the remembering of displays of the boundary sample implemented as reference displays by the former. The inspectors must be well-trained to avoid possible judgment errors.
Further, the reference displays on the sample do not remain unchanged but become fainter with a lapse of time. It is necessary to check the boundary sample regularly in a certain period of time. In short, the periodic checking of the reference displays is required, accordingly.
(2) The inspection method by means of ND filters
Since this method is strongly influenced by ambient illumination at working places, the illumination must be strictly controlled.
In addition, market available ND filters have large dispersion in transmittance. Thus, even if the illumination is strictly controlled, the inspections have dispersion in their results caused by the dispersion in transmittance of the ND filters.
(3) The inspection method by means of software programmed patterns similar to unevenness
Display panels such as liquid crystal display panels have viewing angle characteristics so that the uneven density patterns are subject to sharp changes with respect to viewing angles.